Pseudocapacitive materials

Summarizing the above, it may be stated that activated carbons and pseudocapacitive materials in EC electrode structure are responsible for the energy storage parameters (specific energy), while non-active highly conductive carbon additives are responsible for the electrode internal resistance (EC specific power). 

Among the metal oxide pseudocapacitive materials the most representative are the crystalline ruthenium oxide Ru02 [24] and the amorphous hydrous ruthenium oxide Ru02 XH2O [27, 28], although other materials are under study, for example cobalt oxide [29] and vanadiun oxide [30] xerogels, molybdenum-based materials [31, 32], and Ti-V-W-O oxides [33]. 

The two large families of pseudocapacitive materials under study are conducting polymers [14-17] and metal oxides [8,18-20]. In the case of carbon materials, the surface functionality can also participate to pseudocapacitive redox reactions, as it will be shown later. 

Pseudocapacitors store charge based on reversible (faradaic) charge transfer reactions with ions in the electrolyte. For example, in a metal oxide (such as RUO2 or I1O2) electrode, charge storage results from a sequence of redox reactions. Electrochemical capacitors (ECs) based on such pseudocapacitive materials will have both faradaic and nonfaradaic contributions. The optimization of both EDLCs and pseudocapacitors depends on understanding how features at the nanoscale (e.g. pore size distribution, crystaUite or particle size) affect ion and electron transport and the fundamental properties of electrochemical interfaces. 

Studies on the topic of capacitive storage are oriented toward the increase in energy density of supercapacitors by working on carbon, pseudocapacitive materials and electrolytes. 

BRO 10] Brousse T., Pseudocapacitive materials for supercapacitor applications . International Conference on Advanced Capacitors (ICAC2010), Meeting abstract, Kyoto, Japan, 31 May-2 June 2010. 

This type of pseudocapacitance originates from the redox reactions on some electroactive materials such as metal oxides and hydroxides. RUO2 is the first and one of the most extensively studied redox-type pseudocapacitive material for ESs because of its high specific capacitance from 200 to 1000 F g depending on its structure. The redox reactions of RUO2 in the acid solution involve fast reversible electron transfer accompanied by electro-adsorption of protons on the surface or insertion of protons into the bulk RUO2 [68,72,73] ... 

The ESs mentioned above consist of two electrodes with the same type of capacitive materials made from either EDL capacitive materials or pseudocapacitive materials (symmetrical configuration). In order to further increase the operating potential window, energy, and power density, a new type of ES has been developed, which is known as hybrid capacitors. With extensive achievements in this area, various types of hybrid ESs have been developed. Generally, hybrid capacitors utilize both the EDL capacitance and faradaic reaction to store charges. The hybrid capacitors reviewed in this book include (1) ESs based on composite electrodes made from both EDL capacitive materials and pseudocapacitive materials (2) asymmetric ESs with one EDL electrode and another pseudocapacitive or battery-type electrode and (3) asymmetric ESs with one pseudocapacitive electrode and another rechargeable battery-type electrode. 

The pseudocapacitance can also be provided by other pseudocapacitive materials such as some metal oxides and electrically conductive polymers (ECPs) that have much higher theoretical capacitance than carbon-based materials. These materials have been reviewed in detail elsewhere [89,90]. Although many materials have been reported to exhibit pseudocapacitive behavior, they are very sensitive to the type and pH of the electrolytes and few of them are suitable for application in strong acid electrolytes. As previously mentioned in Section 1.3.2, RUO2 is one of the most extensively studied pseudocapacitive materials in H2SO4 electrolytes. 

Similarly, using C03O4 as pseudocapacitive materials in the KOH electrolyte, the surface faradaic reaction is involved with OH- ions adsorption/desorption or inser-tion/extraction accompanied by the charge transfer process. This can be expressed as follows [127,128] ... 

Furthermore, for pseudocapacitive materials, the cycle stability is a major concern. Besides the repeated ion intercalation/deintercalation-related failure mechanism, it was also found that the decrease of the capacitive performance after long-term charging/discharging cycling might also be related to the dissolution of electrode materials in alkaline electrolytes [142]. Joseph et al. [142] found the cycling stability of Ni3(N03)2(0H)4 in LiOH was higher than that in KOH and NaOH electrolytes. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis showed the evidence of the Ni dissolution into the electrolyte and found the dissolution of... 

Similar mechanisms have been proposed for other metal oxide-based pseudocapacitive materials, such as molybdenum oxide [202,203]. Since ions of electrolytes... 

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